D-psicose has been widely used as a functional sweetener which has a sweet taste similar to sugar but is an ultralow-energy monosaccharide sugar. In particular, D-psicose is known as a “rare sugar” because it is rarely found in nature, and even when found, only in small amounts.
D-psicose is an epimer of D-fructose, and the sweetness intensity and kinds thereof are quite similar to those of D-fructose. However, contrary to D-fructose, since when absorbed into the body, D-psicose is rarely metabolized, it has almost zero calories. Further, due to its action to alleviate abdominal obesity by suppressing enzyme activities regarding to lipid synthesis, D-psicose can be used as an effective ingredient of diet foods. In addition, while sugar alcohols widely used as a substitute for sugar have a problem of causing side effects such as diarrhea when taken in excessive amounts, D-psicose has almost no side effects (Matsue, T., Y. Baba, M. Hashiguchi, K. Takeshita, K. Izumori, and H. Suzuki. 2001. Dietary D-psicose, a C-3 epimer of D-fructose, suppresses the activity of hepatic lipogenic enzymes in rats. Asia Pac. J. Clin. Nutr. 10:233-237; Matsuo, T., and K. Izumori. 2004. D-psicose, a rare sugar that provides no energy and additionally beneficial effects for clinical nutrition. Asia Pac. J. Clin. Nutr. 13:S127).
Due to such reasons, D-psicose is getting the spotlight in the food industry field as a diet sweetener, and thus, there is a growing need to develop a method of producing D-psicose efficiently.
Conventional methods of producing D-psicose by using a psicose-epimerase had been focused to develop an epimerization process of D-psicose from D-fructose by using a recombinant enzyme massively expressed in a recombinant E. coli or a host cell comprising the same. However, the biotechnological production of D-psicose using such a recombinant E. coli is unsuited for the production of D-psicose as a food material in terms of food safety. In order to produce D-psicose suitable to be used as a food material, it is required to develop a mass production method using a psicose-epimerase expressed in a host cell which is a GRAS (Generally Recognized As Safe) strain as well as a strain capable of being industrially mass producing, or using a host comprising the same.
GRAS is granted for substances that are generally recognized to be safe when experts qualified by scientific training and experience evaluate their safety through scientific procedures under the conditions of their intended use. GRAS is a unique system only implemented in USA, but its importance and necessity has been recognized internationally as well as in USA. Therefore, people are paying attention to the development of GRAS.
Meanwhile, for the industrial production of D-psicose, it is required to develop a method of producing D-psicose from a more inexpensive substrate. Currently, due to a high substrate specificity of a psicose-epimerase, only expensive D-fructose is used as a substrate for the production of D-psicose.